Feature request: Add daily treasury yield curve data set

[adamcataldo]

It would be great if this package had daily treasury yield curve data, from: https://home.treasury.gov/resource-center/data-chart-center/interest-rates/TextView?type=daily_treasury_yield_curve

I'd be willing to contribute this myself, if there's someone who can provide any obvious up-front guidance, so I don't spend a bunch of time writing a PR that I end up needing to completely rewrite.